SF: Remove DisplayDeviceState::isValid

SurfaceFlinger::mCurrentState::displays does not contain invalid
DisplayDeviceState, so lookup is sufficient to determine whether
a display token has associated DisplayDeviceState.

Bug: 74619554
Test: libsurfaceflinger_unittest
Change-Id: I618abcb4f0e0f4a808247d59d7d4c4f6ac07a352